We determine deficiency or adequacy of vitamin D by measuring a compound in blood called calcidiol, also called 25-hydroxyvitamin D or 25(OH)D. Calcidiol is not the most biologically potent form of the vitamin. To make that form, our bodies attach another OH group to make calcitriol, also called 1,25-hydroxyvitamin D or 1,25(OH)D.
So, the vitamin D we eat in food, or in a pill, or make in our skin, or measure in our blood is not the active form of the vitamin. And unless we are truly deficient in these precursors (or prehormones), there is no guarantee that increasing any of them will increase levels of this active vitamin D hormone. It is active vitamin D which is responsible for the health benefits accruing from research, not what's in a pill.
Here's a study that supports the notion that taking extra vitamin D isn't helpful, in this case for bone:
Cholecalciferol Supplementation Throughout Winter Does Not Affect Markers Of Bone Turnover In Healthy Young And Elderly Adults, The Journal of Nutrition, March, 2010
"In conclusion, data from these randomized, controlled, double-blind, dose-related intervention trials in healthy 20- to 40-y and ≥64-y-old adults with relatively good calcium intakes would suggest that cholecalciferol [vitamin D] supplementation alone with up to 15 μg/d throughout winter, which achieved mean serum 25(OH)D concentrations of ∼70 nmol/L or more, had no effect on markers of bone turnover."It found no improvement in markers of bone health from taking vitamin D3 ... in levels that at the time surpassed the IOM's recommendation.
By the way, the Vitamin D Council says:1
"It is almost impossible to significantly raise your vitamin D levels when supplementing at only 600 IU/day (15 micrograms)."This study clearly shows a significant increase in vitamin D levels when supplementing at 600 IUs. It also shows an increase in vitamin D levels when supplementing at lesser doses of 200 IUs and 400 IUs. (Remember, these are precursor vitamin D, not active vitamin D.)
Levels Of Active Vitamin D Are Tightly Controlled, Environmental Pollutants May Be Interfering With That Control
Levels of the potent hormonal vitamin D are tightly controlled, hanging around for just a few hours after being made. (That's one reason for measuring levels of the precursor 25(OH)D instead of the more active 1,25(OH)D. The precursor hangs around for several weeks.)
We make active 1,25(OH)D when, for example, we need more calcium since it promotes uptake of calcium from the intestine - we'll absorb more calcium from what we eat. Conversely, high levels of calcium and phosphorus (phos. sources: dairy, fish, meat, soda) inhibit conversion of 25(OH)D to active vitamin D through a feedback loop (managed by parathyroid hormone, see diagram to right and Vitamin D: The Active Form, And The Role Of Calcium)
What else controls how much hormonally active vitamin D we make? Or how efficiently its used? Prednisone and other steroidal drugs such as oral contraceptives. These drugs may impede conversion of calcidiol to calcitriol, or interfere with binding of calcitriol to its carrier protein or receptor. The result is disturbed vitamin D metabolism. Taking too much prednisone leads to lower bone density and osteoporosis.
The interference of steroidal drugs with vitamin D (a product of our endocrine system, also a steroid) has me thinking that other environmental chemicals could be crippling the activity of vitamin D in our body, things like pesticides and bisphenol-A in plastics which are known endocrine disruptors.
The Endocrine Society claims endocrine disruptors are "a significant concern to public health."
"There is growing interest in the possible health threat posed by endocrine-disrupting chemicals (EDCs), which are substances in our environment, food, and consumer products."Endocrine disruptors are found in:
- Endocrine-Disrupting Chemicals: An Endocrine Society Scientific Statement, Endocrine Reviews, June 2009
- Industrial chemicals
- Plastics (such as bisphenol-A)
"Persistent organic pollutants such as dioxins, furans and polychlorinated bromides enter the human food chain through the diets of food animals. The source of these organic compounds for food animals is contaminated forage and soils and the animal feed. Once ingested, the dioxins and other compounds are absorbed and stored in the fat of the animal. By continually re-feeding fat from such animals back to other animals, the dioxins are concentrated more and more, a process called bioaccumulation. When humans consume animal fat in meat and dairy products, they are exposed to these pollutants that are carcinogenic and toxic to the developing nervous system of the foetus and to young children."
- Public Health Implications Of Meat Production And Consumption, Public Health Nutrition, June 2005
In the end, perhaps we don't need to take more vitamin D (which doesn't correlate well with the hormonally active vitamin D anyway); perhaps we need to minimize our exposure to chemicals that interfere with vitamin D metabolism.
A final thought ... Paradoxically, a food we are advised to eat liberally to maintain bone heath is dairy food -- which, of the foods we consume, contains some of the highest levels of calcium, phosphorus, protein, and fat-soluble pollutants including endocrine disruptors. They all conspire to weaken bone, through a reduction of hormonally active vitamin D and an acid load that depletes bone of its minerals. The National Dairy Council knows this but promotes dairy consumption anyway:
"Excess dietary protein, particularly purified proteins, increases urinary calcium excretion. This calcium loss could potentially cause negative calcium balance, leading to bone loss and osteoporosis. These effects have been attributed to an increased endogenous acid load created by the metabolism of protein, which requires neutralization by alkaline salts of calcium from bone."
- National Dairy Council